1. Field of the Invention
This invention relates in general to valves and in particular to an improved pressure relief valve that relieves a pressure of a system fluid by entering an open position and then by returning to the normally-closed position following a delay period.
2. Description of Related Art
In a fluid transport system having a system fluid with a pressure that varies, it is often desirable to relieve or lower the pressure of the system fluid if the pressure reaches an unacceptably high level. The most common way of relieving these high pressures is by use of a pressure relief valve. The pressure relief valve, which is in fluid communication with the system fluid, is designed to detect an unacceptable pressure level and relieve the pressure by opening the valve and allowing the system fluid to escape the system line.
A problem sometimes develops in the use of these valves in systems that experience rapidly varying pressures. For example, in certain oil and gas well treatments, high pressure liquid is pumped down the well to fracture the earth formation. Large high pressure reciprocating pumps at the surface pump the liquid through flow lines leading into the well. In these systems, the pressure in the system lines can become cyclical, with the pressure of the system fluid exceeding and dropping below an acceptable level.
A typical pressure relief valve for use in one of these systems includes a housing having an inlet port and an outlet port. The inlet port is connected to the system line. A valve member is located within the housing that is capable of moving between an open position and a closed position. In the open position, fluid communication is allowed between the inlet port and the outlet port. In the closed position, the valve member sealingly engages a portion of the housing, thereby preventing fluid communication between the inlet port and the outlet port. The valve member is biased such that it remains in the closed position when the pressure of the system fluid is at an acceptable level. As the pressure rises above an acceptable level, the valve member moves to the open position, thereby relieving the pressure by allowing the fluid to flow out of the outlet port of the valve.
A standard pressure relief valve includes a spring which provides a force to bias the valve member into the closed position. The size and type of spring is chosen based on the desired acceptable level of pressure of the system fluid. As the pressure of the system fluid rises above the acceptable level, the force exerted on the valve member by the fluid exceeds the force exerted by the spring, causing the valve member to move to an open position. As soon as the pressure of the system fluid returns to an acceptable level, the force exerted by the fluid becomes less than that exerted by the spring, and the valve member immediately returns to the closed position.
The spring may be mechanical or it may be a compressed gas chamber. A gas cushion spring includes a piston that is connected to one end of the valve member. The piston is disposed within a pressure chamber in the housing, and the piston and the valve member are adapted to move together from the open position to the closed position. A bias or control fluid, which is usually nitrogen gas, is introduced into the pressure chamber above the piston. The pressure of the control fluid exerts a biasing force on the piston, which pushes the piston and the valve member into the closed position. The valve member and piston move to the open position when the force exerted on the valve member by the system fluid exceeds the force exerted on the piston by the control fluid.
Both the mechanical spring and gas spring valves described above provide adequate ventilation of the system fluid when it reaches an unacceptably high pressure. However, both of these valves return immediately to the closed position when the pressure of the system fluid returns to an acceptable level. This method of operation is undesirable when the pressure of the system fluid varies rapidly. A rapid variation of the system fluid pressure causes these standard valves to “chatter,” as they rapidly open and close. The rapid movement of the valve member within the housing causes excessive valve wear and excessive heat to be generated, both of which are undesirable features.
One solution to the “chatter” problem is currently employed in some pressure relief valves. These valves incorporate a manual reset feature that requires an operator to reset the valve once the valve has moved to an open position. Valves of this type typically use a valve member which is biased into the closed position by a mechanical spring. As the pressure of the system fluid rises to an unacceptable level, the valve member moves to an open position. Once it reaches the open position, the valve member is locked until an operator manually resets the valve, allowing the valve member to return the closed position. The problem with this type of valve is that it requires extensive operator monitoring and involvement when the pressure of the system fluid varies rapidly. Additionally, because the valve will not return to a closed position until manually reset, once the valve is opened the system fluid will be expelled from the valve even if the pressure returns to an acceptable level.
U.S. Pat. No. 6,209,561 solved the chatter problem by introducing a pressurized fluid, or delay fluid, beneath the piston to slow the return of the piston and valve member to the closed position. A one-way check valve extends through the piston from the lower portion to the upper portion of the pressure chamber. While returning to the closed position, the fluid in the lower chamber would flow through to the check valve to the upper chamber. While solving the “chatter” problem due to quick returns to the closed position, sometimes pressurized gas would remain in the lower portion of the pressure chamber and prevent the piston and check valve from fully returning to the closed position.